Liquid phase diffusion bonding is a joining technique interposing an insert material having a melting point lower than the joined members, for example, an amorphous metal or amorphous alloy, at the joining surfaces, heating to a temperature higher than a liquidus temperature of the insert material and a temperature lower than the melting point of the joined members, causing the joined parts to melt, and causing isothermal solidification. The amorphous metal, amorphous alloy, or other insert material may, for example, be used in a foil, powder, plating, or other form.
This liquid phase diffusion bonding is applied to joining of stainless steel, high nickel-based alloys, heat resistant steel alloy steels, and other steels difficult to weld by conventional welding methods. Furthermore, according to liquid phase diffusion bonding, it is possible to simultaneously join a large number of locations. Further, when joining members with large cross-sectional areas of the joined parts, the required time does not greatly increase. For this reason, for the purpose of reducing installation costs, liquid phase diffusion bonding is now also being applied even to steel materials able to be joined by welding.
When applying liquid phase diffusion bonding to joining metal pipes, the general practice has been to interpose an insert material constituted by an amorphous metal or amorphous alloy foil between the end faces of the facing metal pipes, apply an axial compressive force to the metal pipes, and while doing so heat the joining surfaces and their vicinities to the liquidus temperature or more.
However, at the time of actual operation, in particular with joining at installation sites, the applied surface pressure becomes unstable. Further, depending on the location of the joining surfaces, the surface pressure becomes low and sufficient joining strength cannot be obtained in some cases.
Furthermore, if the metal pipes are shaped elliptically or are uneven in thickness, the actual joining surfaces will sometimes be reduced from the cross-sectional areas of the joined pipes and the joining strength will deteriorate.
To solve these problems, a joint has been proposed improved in strength by greatly increasing the joining surface with a tapered metal pipe through a coupling or nipple (see for example, PLT 1). Further, a joint has been proposed tapering metal pipes and bonding the metal pipes by liquid phase diffusion bonding (see for example, PLTs 2 and 3).